Neurocognitive and familial moderators of psychiatric risk in velocardiofacial (22q11.2 deletion) syndrome: a longitudinal study

BACKGROUND

Although risk for psychosis in velocardiofacial (22q11.2 deletion) syndrome (VCFS) is well established, the cognitive and familial factors that moderate that risk are poorly understood.

METHOD

A total of 75 youth with VCFS were assessed at three time points, at 3-year intervals. Time 1 (T1) psychiatric risk was assessed with the Behavior Assessment System for Children (BASC). Data reduction of BASC scores yielded avoidance-anxiety and dysregulation factors. Time 2 (T2) neuropsychological and family function and time 3 (T3) prodromal/overt psychosis were assessed. Poisson regression models tested associations between T3 positive prodromal symptoms/overt psychosis and T1 psychiatric risk, T2 cognitive and familial factors, and their interactions.

RESULTS

T1 avoidance-anxiety ratings predicted T3 prodromal/overt psychosis. T2 verbal learning scores moderated this association, such that individuals with low avoidance-anxiety scores and stronger verbal learning skills were the least likely to demonstrate prodromal/overt psychosis at T3. Low scores on a T2 visual vigilance task also predicted T3 prodromal/overt psychosis, independently of the effect of T1 avoidance-anxiety scores. T1 dysregulation scores did not predict T3 prodromal/overt psychosis in a linear manner. Instead, the association between dysregulation and prodromal/overt psychosis was amplified by T2 levels of family organization, such that individuals with low dysregulation scores and low family organization scores were the most likely to exhibit T3 prodromal/overt psychosis.

CONCLUSIONS

Significant moderators of psychiatric risk in VCFS include verbal learning skills as well as levels of family organization, carrying implications for early identification and preventative treatment of youth with VCFS at highest risk for psychosis.

Role of glucocorticoid hormones in arginine vasopressin gene regulation

The mechanism underlying increased AVP synthesis and release in glucocorticoid deficiency is not known. Therefore, the present study was undertaken to investigate whether the mechanism was at the level of AVP gene transcription. The AVP gene promoter contains a consensus GRE, a CRE, and four AP2 sites. To assess the functional importance of these sites, 5' deletions of the AVP promoter were created and transient transfections were performed. Promoter activity in hypothalamic cells transfected with deletions lacking the GRE or both the GRE and CRE exhibited higher activity when compared to longer constructs containing both sites. In neuroblastoma cells, only the deletion lacking the GRE exhibited increased AVP promoter activity over the longer construct. These results are consistent with the idea that glucocorticoids suppress AVP gene expression by acting on a GRE in the AVP promoter region. Further, dexamethasone inhibited AVP promoter activity by >50% in hypothalamic cells transfected with the GRE-containing construct. In conclusion, the data presented here support a central mechanism to explain, at least in part, the nonosmotic increase in AVP with glucocorticoid deficiency.

Signaling pathways responsible for fetal gene induction in the failing human heart: evidence for altered thyroid hormone receptor gene expression

BACKGROUND

We have previously demonstrated that changes in myosin heavy chain (MHC) isoforms that occur in failing human hearts resemble the pattern produced in rodent myocardium in response to hypothyroidism. Because thyroid hormone status is usually within normal limits in these patients, we hypothesized that failing/hypertrophied human myocardium might have a defect in thyroid hormone signaling due to alterations in expression of thyroid hormone receptors (TRs).

METHODS AND RESULTS

To examine this hypothesis, we used RNase protection assay to measure mRNA levels of TRs in failing left ventricles that exhibited a fetal pattern of gene expression, ie, decreased expression of alpha-MHC with increased beta-MHC expression compared with left ventricles from age-matched controls. We detected expression of TR-alpha(1), -alpha(2), and -beta(1) isoforms in human left ventricles. In failing left ventricles, TR-alpha(1) was downregulated, whereas TR-alpha(2), a splice variant that does not bind thyroid hormone but inhibits responses to liganded TRs, was increased. Expression levels of TR-beta(1) did not differ significantly between the 2 groups. According to linear regression analysis, expression levels of TR-alpha(1) and -alpha(2) were positively and negatively correlated with those of alpha-MHC, respectively.

CONCLUSIONS

We conclude that decreases in TR-alpha(1) and increases in TR-alpha(2) may lead to local attenuation of thyroid hormone signaling in the failing human heart and that the resulting tissue-specific hypothyroidism is a candidate for the molecular mechanism that induces fetal gene expression in the failing human ventricle.

The effect of thyroid hormone and a long-acting somatostatin analogue on TtT-97 murine thyrotropic tumors

Thyroid hormone inhibits thyrotropin (TSH) production and thyrotrope growth. Somatostatin has been implicated as a synergistic factor in the inhibition of thyrotrope function. We have previously shown that pharmacological doses of thyroid hormone (levothyroxine [LT4]) inhibit growth of murine TtT-97 thyrotropic tumors in association with upregulation of somatostatin receptor type 5 (sst5) mRNA and somatostatin receptor binding. In the current study, we examined the effect of physiological thyroid hormone replacement alone or in combination with the long-acting somatostatin analogue, Sandostatin LAR, on thyrotropic tumor growth, thyrotropin growth factor-beta (TSH-beta), and sst5 mRNA expression, as well as somatostatin receptor binding sites. Physiological LT4 replacement therapy resulted in tumor shrinkage in association with increased sst5 mRNA levels, reduced TSH-beta mRNA levels and enhanced somatostatin receptor binding. Sandostatin LAR alone had no effect on any parameter measured. However, Sandostatin LAR combined with LT4 synergistically inhibited TSH-beta mRNA production and reduced final tumor weights to a greater degree. In this paradigm, Sandostatin LAR required a euthyroid status to alter thyrotrope parameters. These data suggest an important interaction between the somatostatinergic system and thyroid hormone in the regulation of thyrotrope cell structure and function.

Cloning of the mouse somatostatin receptor subtype 5 gene: promoter structure and function

Somatostatin is a peptide hormone whose actions are mediated by five somatostatin receptor subtypes (sstl-5). In the pituitary, somatostatin inhibits TSH release from thyrotropes and GH release from somatotropes. We have shown that sst5 transcripts and protein are induced by thyroid hormone in TtT-97 thyrotropic tumors. To map sequences responsible for promoter activity in pituitary cells, we cloned the mouse sst5 coding region of 362 amino acids and 12 kb of upstream DNA. Initial transfection studies in TtT-97 or GH3 cells mapped high levels of basal promoter activity to a 5.6-kb fragment upstream of the translational start, whereas shorter genomic fragments had low activity. To identify the transcriptional start site we used 5' RACE with TtT-97 poly A+ RNA and a sst5 antisense coding region primer. Sequence comparison between the complementary DNA and the gene revealed that the mouse sst5 gene contains 3 exons and 2 introns. The entire coding region was contained in exon 3. Two differently sized RACE products demonstrated alternate exon splicing of two untranslated exons in TtT-97 cells. A promoter fragment from -290/+48 linked to a luciferase reporter demonstrated 600- and 900-fold higher activity over a promoterless control in GH3 mammosomatotropes and TtT-97 thyrotropes, respectively, whereas a larger fragment extending to -6400 exhibited no additional promoter activity. Cloning of the sst5 gene will facilitate the mapping of basal and regulated responses at the transcriptional level.

Reverse transcription polymerase chain reaction analysis of pituitary hormone, Pit-1 and steroidogenic factor-1 messenger RNA expression in pituitary tumors

Pit-1 is a transcription factor that appears early in embryonic pituitary gland formation and is necessary for the development of somatotropes, lactotropes and thyrotropes. Steroidogenic factor-1 (SF-1) is another early appearing transcription factor that is involved in the development of gonadotropes. In this study we have compared RT-PCR analysis of hormone mRNA with traditional IHC for classification of 27 pituitary tumors and have evaluated the correlation of Pit-1 and SF-1 mRNA with hormone mRNA. RT-PCR detected concordant hormone mRNA in 100% of GH IHC positive, 100% of PRL IHC positive, 33% of TSH IHC positive, and 93% of gonadotropin IHC positive tumors. IHC, however, was concordant in only 71% of GH mRNA positive, 78% of PRL mRNA positive, 17% of TSH beta mRNA positive, and 76% of FSH beta mRNA positive tumors. Pit-1 mRNA was positive in 87% of tumors in which mRNA for GH, PRL or TSH beta was detected and in only 17% of GH, PRL and TSH beta mRNA negative tumors. SF-1 mRNA was positive in 94% of tumors in which mRNA for FSH beta was present and in no FSH beta mRNA negative tumors. We conclude that RT-PCR analysis of hormone mRNA may be more sensitive than traditional hormone IHC for classification of pituitary tumors. Furthermore, tumor Pit-1 mRNA positively correlates with GH, PRL and TSH beta mRNA while tumor SF-1 mRNA correlates well with FSH beta mRNA. Combined analysis of hormone and transcription factor mRNA in pituitary tumor tissue may therefore be a more meaningful approach to pituitary tumor characterization.

An upstream regulator of the glycoprotein hormone alpha-subunit gene mediates pituitary cell type activation and repression by different mechanisms

Targeting of alpha-subunit gene expression within the pituitary is influenced by an upstream regulatory region that directs high level expression to thyrotropes and gonadotropes of transgenic mice. The same region also enhanced the activity of the proximal promoter in transfections of pituitary-derived alpha-TSH and alpha-T3 cells. We have localized the activating sequences to a 125-bp region that contains consensus sites for factors that also play a role in proximal promoter activity. Proteins present in alpha-TSH and alpha-T3 cells as well as those from GH3 somatotrope-derived cells interact with this region. The upstream area inhibited proximal alpha-promoter activity by 80% when transfected into GH3 cells. Repression in GH3 cells was mediated through a different mechanism than enhancement, as supported by the following evidence. Reversing the orientation of the area resulted in a loss of proximal promoter activation in alpha-TSH and alpha-T3 cells but did not relieve repression in GH3 cells. Mutation of proximal sites shown to be important for activation had no effect on repression. Finally, bidirectional deletional analysis revealed that multiple elements are involved in activation and repression and, together with the DNA binding studies, suggests that these processes may be mediated through closely juxtaposed or even overlapping elements, thus perhaps defining a new class of bifunctional gene regulatory sequence.

Activation of the glycoprotein hormone alpha-subunit gene promoter in thyrotropes

Pituitary expression of the glycoprotein hormone alpha-subunit gene localizes to thyrotropes and gonadotropes. Factors that activate the alpha-subunit promoter exclusively in thyrotropes have not been described. In the current studies, alpha-subunit promoter activity was compared in alphaTSH thyrotropic and alphaT3 gonadotropic cells. Deletional analysis revealed a more important contribution of the -453/-381 region in thyrotropic cells and the -341/-297 region in gonadotropic cells, while other deletions had similar effects. Mutational analysis revealed some regions that were functionally similar, while others were active only in thyrotropic cells, such as the regions - 434/ - 421, - 398/ - 385, - 376/- 364 and - 363/- 351, that decreased activity by approximately 2-fold each. Combined mutation of the regions - 434/ - 421, - 398/ - 385 and - 376/ - 364 decreased activity by 5-fold in thyrotropic cells and not at all in gonadotropic cells. Cotransfection with Ptx1 in CV-1 cells resulted in a strong stimulation of alpha-subunit promoter activity, that was significantly diminished with mutation of the - 398/ - 385 region that disrupts the Ptx1 binding site, suggesting that this factor may play a role in thyrotrope-specific activation. This analysis provides basic information important to identify common and unique factors contributing to the cell-specific expression of the alpha-subunit gene.

Functional interactions of an upstream enhancer of the mouse glycoprotein hormone alpha-subunit gene with proximal promoter sequences

Transcription of the glycoprotein hormone alpha-subunit gene in the pituitary is governed by different promoter elements in thyrotropes and gonadotropes. We recently identified an upstream enhancer that directs a high level of cell type specific expression in transgenic mice and stimulates proximal promoter activity in cultured alphaTSH and alphaT3 cells. To assess the contribution of promoter sequences that functionally interact with the enhancer, we mutated two proximal elements shown to be important in both thyrotrope and gonadotrope cells. Disruption of the pituitary glycoprotein hormone basal element (PGBE), which binds a LIM homeodomain protein, resulted in a decrease in basal promoter activity in both alphaTSH and alphaT3 cells. Enhancer function was completely abolished by the PGBE site mutation in alphaT3 gonadotropes, whereas some stimulatory activity remained in alphaTSH thyrotropes. Mutation of the gonadotrope specific element (GSE), which binds SF1 and is important for basal activity in gonadotropes and TRH response in thyrotropes, resulted in declines in basal and enhanced promoter activity only in alphaT3 cells and not in alphaTSH cells. Despite this decrease in enhanced activity, the GSE mutated promoter still retained some enhancer stimulated activity, suggesting that the PGBE site still functionally interacts in the absence of an intact GSE. This mutation had no effect in alphaTSH cells. These data suggest that although the enhancer works in both cell types it exhibits cell type specific functional characteristics.

Cell-specific expression of the mouse glycoprotein hormone alpha-subunit gene requires multiple interacting DNA elements in transgenic mice and cultured cells

The glycoprotein hormone alpha-subunit gene is expressed and differentially regulated in pituitary gonadotropes and thyrotropes. Previous gene expression studies suggested that cell specificity may be regulated by distinct DNA elements. We have identified an enhancer region between -4.6 and -3.7 kb that is critical for high level expression in both gonadotrope and thyrotrope cells of transgenic mice. Fusion of the enhancer to -341/+43 mouse alpha-subunit promoter results in appropriate pituitary cell specificity and transgene expression levels that are similar to levels observed with the intact -4.6 kb/+43 construct. Deletion of sequences between -341 and -297 resuited in a loss of high level expression and cell specificity, exhibited by ectopic transgene activation in GH-, ACTH-, and PRL-producing pituitary cells as well as in other peripheral tissues. Consistent with these results, transient cell transfection studies demonstrated that the enhancer stimulated activity of a -341/+43 alpha-promoter in both alphaTSH and alphaT3 cells, but it did not enhance alpha-promoter activity significantly in CV-1 cells. Removal of sequences between -341 and -297 allowed the enhancer to function in heterologous cells. Loss of high level expression and cell specificity may be due to loss of sequences required for binding of the LIM homeoproteins or the alpha-basal element 1. These data demonstrate that the enhancer requires participation by both proximal and distal sequences for high level expression and suggests that sequences from -341 to -297 are critical for restricting expression to the anterior pituitary.

Arginine vasopressin secretion with mutants of wild-type and Brattleboro rats AVP gene

Defects in peptide processing are associated with several disorders, including central diabetes insipidus (CDI). In the Brattleboro (BB) rat with CDI, the mRNA and protein of arginine vasopressin (AVP) are present in the hypothalamus, but no circulating AVP is detectable, thus suggesting a processing defect. The present study examined AVP secretion in cultured COS cells transfected with various constructs from wild-type and mutated Brattleboro AVP gene precursors. The precursor contains three exons encoding for vasopressin (VP), neurophysin (NP), and glycopeptide (GP). The Brattleboro rat has a deletion of a single base, guanine (G), in the NP coding region that leads to a frameshift, resulting in the loss of normal stop codon. The wild-type pcVP (22.0 +/- 5.2 pg/10[-2] U beta-galactosidase [beta-gal]), but not the mutated BB AVP gene pcBB (1.2 +/- 0.4 pg/10[-2] U beta-gal), was associated with AVP secretion from the COS cells as measured by RIA. The wild-type AVP gene without the GP coding region was associated with AVP release greater (47.4 +/- 13.5 pg/10[-2] U beta-gal, n = 5, P < 0.05, versus pcVP) than the pcVP with intact VP, NP, and GP coding regions. However, the wild-type AVP gene with VP coding region alone was not processed and secreted. Normalizing the pcBB total length with the insertion of a stop codon at the site of the normal stop codon was not associated with AVP secretion (3.0 +/- 1.4 pg/10[-2] U beta-gal). However, insertion of a stop codon so that the pcBB length equaled the length of VP and NP coding regions of the wild type was associated with AVP secretion (13.5 +/- 4.0 pg/10[-2] U beta-gal). When a stop codon was inserted into the wild-type NP coding region at the same site as the G deletion in the pcBB, the AVP secretion was significantly lower (15.1 +/- 5.0 pg/10[-2] U beta-gal) than pcVP with VP + NP but no GP coding regions (47.4 +/- 13.5 pg/10[-2] U beta-gal, n = 5, P < 0.05). In summary, (1) both VP and intact NP, but not GP, coding regions are necessary for AVP processing and secretion; (2) decreasing the length of the NP coding region diminishes but does not abolish AVP processing and secretion; and (3) shortening of the pcBB length with a stop codon at a site comparable to wild-type VP + NP allows AVP secretion, albeit less than with wild-type gene precursor. Thus, the CDI in BB rats is caused by the G deletion in NP coding region. This defect leads to abnormalities that contribute to the abnormal AVP processing. Specifically, the frameshift and absence of a stop codon cause a mutated extended C terminus, which, along with the mutated NP, contribute to the abnormal steps of AVP processing, transport, and secretion in the BB rat. These defects no doubt impair the folding and configuration necessary for normal processing of the AVP gene precursor.

Msx1 is present in thyrotropic cells and binds to a consensus site on the glycoprotein hormone alpha-subunit promoter

Our studies are aimed at identifying the transcription factors that activate the glycoprotein hormone alpha-subunit promoter. Therefore, we performed a Southwestern screening of a thyrotropic (alphaTSH) cDNA expression library, using the region of the promoter from -490 to -310 that contains sequences critical for expression in thyrotrope cells. A clone was isolated corresponding to part of the coding sequence of Msx1, which is a homeodomain-containing transcription factor that has been found to play an important role in the development of limb buds and craniofacial structures. Northern blot analysis, using the cloned Msx1 cDNA fragment as a probe, demonstrated that alpha-subunit-expressing thyrotrope cells (alphaTSH cells and TtT97 tumors) contained Msx1 RNA transcripts of 2.2 kb, while somatomammotrope (GH3) cells that do not produce the alpha-subunit had barely detectable levels. The presence of Msx1 protein was demonstrated by Western blot analysis in alphaTSH cells. We also demonstrated that transcripts encoding the closely related Msx2 factor were not detectable by Northern blot analysis in either thyrotrope or somatomammotrope-derived cells. Subfragments of the region from -490 to -310 of the alpha-subunit promoter were used in a Southwestern blot assay using bacterially produced Msx1 and demonstrated that binding was localized specifically to the region from -449 to -421. Deoxyribonuclease I protection analysis, using purified Msx1 homeodomain, demonstrated structurally induced differences in DNA digestion patterns between -436 and -413, and sequence analysis of this region revealed a direct repeat of the sequence GXAATTG, which is similar to the Msx1 consensus-binding site. When nucleotides at both sites were mutated, Msx1 binding was dramatically reduced, and the activity of an alpha-subunit promoter construct decreased by approximately 50% in transfected thyrotrope (alphaTSH) cells. These studies suggest that Msx1 may play a role in the expression of the alpha-subunit gene in thyrotrope cells.

Pit-1 and GATA-2 interact and functionally cooperate to activate the thyrotropin beta-subunit promoter

The molecular determinants governing cell-specific expression of the thyrotropin (TSH) beta-subunit gene in pituitary thyrotropes are not well understood. The P1 region of the mouse TSHbeta promoter (-133 to -88) region interacts with Pit-1 and an additional 50-kDa factor at an adjacent site that resembles a consensus GATA binding site. Northern and Western blot assays demonstrated the presence of GATA-2 transcripts and protein in TtT-97 thyrotropic tumors. In electrophoretic mobility shift assays, a comigrating complex was observed with both TtT-97 nuclear extracts and GATA-2 expressed in COS cells. The complex demonstrated binding specificity to the P1 region DNA probe and could be disrupted by a GATA-2 antibody. When both Pit-1 and GATA-2 were combined, a slower migrating complex, indicative of a ternary protein-DNA interaction was observed. Cotransfection of both Pit-1 and GATA-2 into CV-1 cells synergistically stimulated mouse TSHbeta promoter activity 8.5-fold, while each factor alone had a minimal effect. Mutations that abrogated this functional stimulatory effect mapped to the P1 region. Finally, we show that GATA-2 directly interacts with Pit-1 in solution. In summary, these data demonstrate functional synergy and physical interaction between homeobox and zinc finger factors and provide insights into the transcriptional mechanisms of thyrotrope-specific gene expression.

Progesterone regulated expression of flavin-containing monooxygenase 5 by the B-isoform of progesterone receptors: implications for tamoxifen carcinogenicity

Progesterone is a key developmental, proliferative, and differentiative hormone in the breast and endometrium, and it can accelerate carcinogenesis in the mammary gland epithelium. In the breast and uterus, progesterone acts through two coexpressed isoforms of progesterone receptors, the B- and A-receptors. To study the function of each isoform in isolation, we previously constructed two breast cancer cell lines that stably and independently express either B-receptors (YB cells) or A-receptors (YA cells). In the present study, YA or YB cells were left untreated, or were treated with the synthetic progestin R5020, and the messages present in each cell line under the two conditions were analyzed by differential display. Two message species are described that are regulated only by B-receptors. One of these is regulated in a ligand-independent manner. A third set of messages, encoding flavin-containing monooxygenase 5 (FMO5), was induced by R5020 only in YB cells. A-receptors appear to be inhibitory. FMOs are involved in the metabolic activation of drugs and xenobiotic compounds, including the antiestrogen tamoxifen, to carcinogenic intermediates. It is possible, therefore, that by upregulating the levels of FMO5, progesterone enhances the carcinogenicity of tamoxifen in target tissues that overexpress progesterone B-receptors.

Reconstitution of triiodothyronine inhibition in non-triiodothyronine-responsive thyrotropic tumor cells using transfected thyroid hormone receptor isoforms

The triiodothyronine (T3) inhibitory effect on the thyrotropin (TSH)beta- and alpha-subunit genes is believed to be mediated by binding of T3 to specific nuclear receptors that are present in various isoforms. alphaTSH cells, which are derived from a pure alpha-subunit secreting thyrotropic tumor, contain the same nuclear factors that are important for alpha-subunit gene expression in TSH-expressing T3-responsive thyrotropic cells (TtT97). However, as in the parent tumor, alpha-subunit expression in alphaTSH cells was not inhibited by T3, despite the presence of high-affinity nuclear T3 receptors (TRs) with a similar number of sites per cell as in TtT97. When transcripts coding for the different TR isoforms from the MGH101A tumor were analyzed by Northern blot, TR alpha1 was present, as well as the non-T3-binding variant alpha2, but transcripts encoding the opposite strand Rev-ErbAa were not detectable and neither TR beta1 nor TR beta2 mRNAs were detectable, whereas all these transcripts were detectable in TtT97 tumors. Similar findings were observed in alphaTSH cells, where TR beta1 transcripts were barely detectable in Northern blots and TR beta2 transcripts were detectable only by RT-PCR. The TR beta gene locus is present and unrearranged in the tumor genome. In transient transfection studies conducted in alphaTSH cells overexpression of either TR beta1, TR beta2, or TR alpha1 reconstituted T3-inhibition of the alpha-subunit promoter down to 40% to 50% of control. We conclude that the relative lack of TR beta gene expression correlates with unresponsiveness to T3. The alphaTSH cell line represents a unique model in which to dissect the mechanism of T3 inhibition.

The thyrotrope-restricted isoform of the retinoid-X receptor-gamma1 mediates 9-cis-retinoic acid suppression of thyrotropin-beta promoter activity

TSHbeta is a subunit of TSH that is uniquely expressed and regulated in the thyrotrope cells of the anterior pituitary gland. Thyroid hormone receptors (TR) are known to mediate T3 suppression of TSHbeta gene expression at the level of promoter activity. The role of other nuclear receptors in regulation of this gene is less clearly defined. Retinoid X receptors (RXR) are a family of nuclear transcription factors that function both as 9-cis-retinoic acid (RA) ligand-dependent receptors and heterodimeric partners with TR and other nuclear receptors. Recently, the RXR isoform, RXRgamma, has been identified in the anterior pituitary gland and found to be restricted to thyrotrope cells within the pitutiary. In this report, we have further characterized the distribution of RXRgamma1, the thyrotrope-restricted isoform of RXRgamma, in murine tissues and different cell types. We have found that RXRgamma1 mRNA and protein are expressed in the TtT-97 thyrotropic tumor, but not the thyrotrope-variant alphaTSH cells or somatotrope-derived GH3 cells. Furthermore, we have studied the effects of RXRgamma1 on TSHbeta promoter activity and hormone regulation in these pituitary-derived cell types. Both T3 and 9-cis-RA independently suppressed promoter activity in the TtT-97 thyrotropes. Interestingly, the combination of ligands suppressed promoter activity more than either alone, indicating that these hormones may act cooperatively to regulate TSHbeta gene expression in thyrotropes. The RXRgamma1 isoform was necessary for the 9-cis-RA-mediated suppression of TSHbeta promoter activity in alphaTSH and GH3 cells, both of which lack this isoform. RXRbeta, a more widely distributed isoform, did not mediate these effects. Finally, we showed that the murine TSHbeta promoter region between -200 and -149 mediated a majority of the 9-cis-RA suppression of promoter activity in thyrotropes. This region is distinct from the T3-mediated response region near the transcription start site. These data suggest that retinoids can mediate TSHbeta gene regulation in thyrotropes and the thyrotrope-restricted isoform, RXRgamma1, is required for this effect.

Thyroid hormone-induced expression of specific somatostatin receptor subtypes correlates with involution of the TtT-97 murine thyrotrope tumor

Following the protracted hypothyroid state, treatment with thyroid hormone will induce a decline in TSH and reduce thyrotrope hyperplasia. Somatostatin is a hypothalamic peptide that has been implicated in the negative regulation of TSH secretion in the thyrotrope. Moreover, analogs of native somatostatin have potent TSH-reducing and growth-retarding effects on human thyrotropinomas. The TtT-97 tumor is an in vivo murine thyrotropic model that has retained its physiological response to thyroid hormone. This study investigates the regulation of somatostatin receptor subtypes in this tumor. TtT-97 tumors, actively growing in hypothyroid mice, did not express any significant somatostatin receptor messenger RNA (mRNA) or protein. T4 administration resulted in a reduction in TSH beta mRNA expression and a marked degree of tumor involution. Analysis of residual tumors from thyroid hormone-treated mice showed the specific up-regulation of SSTR1 and SSTR5 mRNA subtypes and the appearance of abundant, high affinity SSTR receptor-binding sites within the tumor. Thus, the TtT-97 tumor provides a thyrotrope-specific model in which to study the regulation of somatostatin receptor subtypes by thyroid hormone and correlate this expression with both antisecretory and antiproliferative effects.

Osmotic and non-osmotic regulation of arginine vasopressin (AVP) release, mRNA, and promoter activity in small cell lung carcinoma (SCLC) cells

Arginine vasopression (AVP) is synthesized in the magnocellular neurons of the hypothalamus and stored in the posterior pituitary. It has been shown that hypothalamic AVP mRNA is increased during experimental stimulation of osmotic and non-osmotic stimulation of AVP release. The mechanisms underlying the stimulation of AVP biosynthesis in these conditions are not known. The present study was, therefore, performed to measure AVP release, AVP mRNA level, and AVP gene promoter activity during osmotic and non-osmotic stimulation of AVP secretion in the small cell lung carcinoma (SCLC) cells. AVP release was measured by radioimmunoassay, steady state levels of AVP mRNA by solution hybridization, and AVP gene promoter activity exhibited by a 1.5 kb 5'-flanking AVP gene fragment fused to a luciferase reporter after SCLC cells were subjected to osmotic or non-osmotic conditions. High media osmolality (330 mOsm) significantly increased AVP release (control (C) 1.42 +/- 0.27 vs. High Osm 3.67 +/- 0.39 pg/2 x 10(6) cells, N = 9, P < 0.002); AVP mRNA (C 173.6 +/- 16.8 vs. High Osm 280.1 +/- 19.4 pg/2 x 10(6) cells, N = 7, P < 0.001); and AVP gene promoter activity (C 1353 +/- 99 vs. High Osm 2026 +/- 134 L.U./10(-4) U beta-gal, N = 8, P < 0.001). Non-osmotic stimulators. 0.1 microM endothelin 3 (ET3), 1 microM angiotensin II (AII), and 10 microM acetylcholine (Ach) significantly increased AVP release; ET3 (C 1.78 +/- 0.20 vs. ET3 6.85 +/- 1.86 pg/2 x 10(6) cells, N = 8, P < 0.02); AII (C 1.29 +/- 0.38 vs. AII 27.80 +/- 7.09 pg/2 x 10(6) cells, N = 5, P < 0.05) and Ach (C 1.14 +/- 0.33 vs. Ach 2.68 +/- 0.58 pg/2 x x10(6) cells, N = 6, P < 0.05). However, only ET3 significantly increased AVP mRNA (C 166.6 +/- 19.6 vs. ET3 254.4 +/- 25.6 pg/p x 10(6) cells, N = 5, P < 0.05) and AVP promoter activity (C 1515 +/- 163 vs. ET3 2389 +/- 342 L.U./10(-4) U beta-gal, N = 6, P < 0.05). To localize the region of the AVP promoter that mediates the osmotic stimulation and the effect of ET3, 5' deletions of the AVP promoter fragments terminating at -532, -211, and -102, was assessed. Only the promoter activity of the 1.5 kb construct, but not the deletion constructs, was significantly increased by ET3 or high osmolality. These results suggest that modulation of AVP gene transcription is, at least in part, responsible for increased AVP synthesis and release in response to osmotic and non-osmotic stimulation, and that the region of 5' flanking sequence between -1500 and -532 contains the elements responsible for the effects.

Insulin internalization in the absence of the insulin receptor tyrosine kinase domain is insufficient for mediating intracellular biological effects

The intracellular portion of the human insulin receptor (hIR) beta-subunit contains distinct functional domains including the exon 16-encoded juxtamembrane (JM) domain that mediates endocytosis, and the tyrosine kinase (TK) domain that mediates insulin's metabolic and mitogenic actions. To explore the functional relationship between these domains and to determine the role of insulin internalization in insulin action, we constructed and studied the endocytic and signaling properties of an hIR mutant truncated at Glu-1012. This truncation removes the carboxyl-terminal 343 amino acids containing essentially all of the TK domain but leaves behind the exon 16-encoded JM domain that is necessary for endocytosis, plus an additional 35 amino acids downstream. The wild-type (hIR-WT) and mutant (hIR delta 343) receptors were stably expressed in CHO cells and their abilities to mediate various insulin-stimulated functions were comparatively analyzed. In cells expressing hIR-WT, insulin markedly enhanced tyrosine phosphorylation of the beta-subunit and of the endogenous 185 kDa substrate whereas these effects were completely absent in cells expressing hIR delta 343. The hIR delta 343 receptors retained the ability to internalize a significant amount of surface-bound insulin at 37 degrees C. However, they were unable to mediate either the short or long-term biological effects of insulin as determined by assaying insulin-stimulated glucose uptake (assessed by 2-deoxyglucose uptake), protooncogene expression (measured by Northern blot analysis of c-fos mRNA) and DNA synthesis (measured by 3H-thymidine incorporation). These results indicate that the hIR beta-subunit JM and TK domains can be functionally uncoupled, and that insulin internalization in the absence of hIR TK domain and kinase activity is insufficient for mediating intracellular insulin action.

Thyroid hormone receptor beta2 promoter activity in pituitary cells is regulated by Pit-1

There are three known thyroid hormone receptor (TR) isoforms that arise from two distinct alpha and beta gene loci. TRalpha1 and TRbeta1 mRNAs are found in many tissues, whereas mRNA for the N-terminal TRbeta2 variant derived from the beta locus is readily detectable only in the pituitary gland and derived cell sources such as GH3 somatotropes and TtT-97 thyrotropes. We previously isolated the genomic region governing expression of the TRbeta2 isoform in thyrotropes and showed that transcription arose from multiple origins within a 400-base pair (bp) region. We now report that the region extending 500 bp upstream of the putative AUG codon (A is +1) contains six areas of interaction with the pituitary-specific transcription factor Pit-1. In addition there are separate areas that bind other factors present in thyrotrope cells. Promoter deletions revealed that removal of regions containing the Pit-1 sites at -456 to -432, -149 to -127, and -124 to -102 progressively decreased TRbeta2 promoter activity in thyrotropes. A more proximal footprinted area from -65 to -19, which accounted for the remaining promoter activity, contained sites that interacted with recombinant Pit-1; however, extracts of TtT-97 thyrotropes, which express Pit-1, footprinted this proximal region with a pattern of protection that differed from that produced by Pit-1. A comparative deletional analysis demonstrated that a shorter region extending only 204 bp from the AUG was sufficient to support TRbeta2 promoter activity in GH3 somatotropes. The more proximal Pit-1 sites, including the area from -53 to -19, whose pattern differed from Pit-1 in thyrotrope extracts, showed protection patterns with GH3 extracts that were indistinguishable from recombinant Pit-1. Site-directed mutagenesis that abrogated binding of both recombinant Pit-1 and Pit-1-containing nuclear extracts revealed that the two Pit-1 sites between -149 and -102 were important for TRbeta2 promoter activity with the more proximal being most critical. Finally, we showed that TRbeta2 promoter activity in alpha-TSH cells, which do not transcribe the endogenous TRbeta2 locus or produce Pit-1 protein, could be reconstituted to a level approaching that seen in expressing TtT-97 thyrotropes by cotransfecting a Pit-1 expression vector. Activation by Pit-1 was dependent on the same Pit-1 sites shown to be important for basal TRbeta2 promoter activity in thyrotropes as constructs lacking them by deletion or mutation were not stimulated by Pit-1.

Determinants of thyrotrope-specific thyrotropin beta promoter activation. Cooperation of Pit-1 with another factor

Thyrotropin (TSH) beta is a subunit of TSH, the expression of which is limited to the thyrotrope cells of the anterior pituitary gland. We have utilized the thyrotrope-derived TtT-97 thyrotropic tumors to investigate tissue-specific expression of the TSH beta promoter. TSH beta promoter activity in thyrotropes is conferred by sequences between -270 and -80 of the 5'-flanking region. We have recently reported that the proximal region from -133 to -100 (P1) is required for promoter expression in thyrotropes. This region interacts with the pituitary-specific transcription factor Pit-1. While Pit-1 appears necessary for TSH beta promoter activity in thyrotropes, this transcription factor is not alone sufficient for promoter activity in pituitary-derived cells. In this report, we have generated a series of promoter mutations in the P1 region to identify additional protein-DNA interactions and determine their functional significance. We have found that Pit-1 interacts with the distal portion of the P1 region, and a second protein interacts with the proximal segment of this region. Each protein is able to independently interact with the TSH beta promoter, but neither alone can maintain promoter activity. Both proteins appear to be necessary for full promoter activity in thyrotropes. Southwestern analysis with the proximal segment of the P1 region (-117 to -88) reveals interaction with a 50-kDa protein. Interestingly, this protein is not found in the pituitary-derived GH3 cells and may represent a thyrotrope-specific transcription factor. Further characterization of this newly identified DNA-binding protein will further our understanding of the tissue-specific expression of the TSH beta gene.

N-terminal variants of thyroid hormone receptor beta: differential function and potential contribution to syndrome of resistance to thyroid hormone

The human syndrome of resistance to thyroid hormone (RTH) is associated with dominant mutations in the thyroid hormone receptor beta (TR beta) gene that generate mutant receptors with impaired binding for T3. Although the TR beta gene differentially expresses two N-terminal variant receptors, TR beta 1 and TR beta 2, functional analyses of RTH mutants have focused exclusively on TR beta 1. Since TR beta 2 is expressed in tissues that are malfunctional in RTH, the role of mutations in the context of TR beta 2 was examined. We compared the functional properties of corresponding RTH mutations in the common C-terminal domain of both TR beta 1 and TR beta 2. Wild type TR beta 1 and TR beta 2 bound similarly as homodimers and as heterodimers with retinoid X receptors to T3-responsive elements consisting of a direct repeat with 4-base pair spacing or an everted repeat. Homodimers, but not monomers or heterodimers, of both receptor subtypes were dissociated by the addition of T3. However, TR beta 2 formed at least 10-fold more stable homodimers than TR beta 1 on a palindromic repeat element, indicating that the N termini of TR beta 1 and TR beta 2 differentially influence dimerization on DNA. The RTH-like mutants of both TR beta 1 and TR beta 2 were equally insensitive to T3. They were defective in T3 binding but still bound DNA like their wild type counterparts except that the T3-dependent dissociation of homodimers from DNA was severely reduced. Wild type TR beta 1 and TR beta 2 mediated T3-inducible transactivation in cotransfection assays; this, however, was abolished in both mutants. TR beta 1 mediated more sensitive T3-dependent transcriptional suppression than TR beta 2 through the negative T3 response region of the TSH beta gene. Again, the mutation abolished T3-dependent suppression by both mutants. Furthermore, both mutants inhibited T3-inducible transcriptional activation by different wild type TR alpha and beta variants. These results indicate that both mutants have the potential to contribute to the pathogenesis of RTH and suggest that a reassessment of previous models of RTH is required to take into account the inhibitory activity of both TR beta 2 and TR beta 1 mutants.

The amino acid sequence GPLY is not necessary for normal endocytosis of the human insulin receptor B isoform

The human insulin receptor (hIR) cytoplasmic juxtamembrane domain contains two tyrosine (Y) residues which exist in GPLY and NPEY motifs that have been implicated in endocytic function. We have previously shown that the NPEY motif is not necessary for endocytosis of the B isoform (exon 11+) of hIR. To examine the role of the GPLY sequence in transmembrane insulin signaling and endocytic functions of hIR-B, we constructed a mutant receptor, hIR delta GPLY, that lacks the GPLY sequence (residues 962-965), and stably expressed it in CHO cells. When compared to wild type hIR-B (hIR-WT) similarly expressed in CHO cells, the hIR delta GPLY mutant exhibited higher insulin binding affinity (EC50 of 1.0 vs 3.5 nM) and normal insulin-stimulated receptor tyrosine autophosphorylation and kinase activity towards the endogenous 185 kDa insulin receptor substrate. The hIR delta GPLY receptor also exhibited normal endocytic functions as hIR-WT in that: a) the internalization of surface photoaffinity labeled hIR delta GPLY was similar to that of hIR-WT, and b) the rate and extent of 125I-insulin internalization and degradation at 37 degrees C were also unimpaired. Therefore, these results demonstrate that the GPLY sequence is not necessary for transmembrane insulin signaling and endocytic functions of the hIR-B isoform.

Structural and functional characterization of the genomic locus encoding the murine beta 2 thyroid hormone receptor

beta 1 and beta 2 are functional thyroid hormone receptors (TRs) that are generated from the same genomic locus by splicing of a different amino terminus onto a common carboxyl region containing the DNA and hormone binding domains. TR beta 1 is widely expressed whereas TR beta 2 is found primarily in the pituitary gland although low levels of expression have been described in other tissues. To gain insight into the mechanisms governing expression of this complex transcriptional unit, we cloned mouse genomic fragments containing the common carboxyl terminus as well as the unique TR beta 2 amino-terminal sequence that was located at least 25 kilobases upstream. The DNA and ligand binding exons are identical in size and location of their boundaries to those of the human TR beta 1 gene. To determine whether the region 5' of the TR beta 2 amino terminus represented the promoter region, we examined it for sites of transcriptional initiation and for its ability to function as a promoter in TR beta 2-expressing thyrotrope cells. Multiple transcriptional start sites extending over 400 base pairs (bp) were identified with those more proximal showing inhibition by T3. Transcription was not detected more than 400 bp upstream from the putative AUG codon, although initiation downstream of this AUG was demonstrated indicating alternative AUG usage. A fragment containing 500 bp of the TR beta 2 5'-region exhibited preferential promoter activity when transfected into thyrotrope cells that express endogenous TR beta 2. Deletion studies demonstrated that removal of consensus binding sites for the transcription factor Pit-1 resulted in loss of this cell specificity. We therefore conclude that the promoter region responsible for expression of the TR beta 2 isoform in pituitary thyrotropes is distinct from that described for TR beta 1 and is located many kilobases upstream from their common exons.

The combination of Pit-1 and Pit-1T have a synergistic stimulatory effect on the thyrotropin beta-subunit promoter but not the growth hormone or prolactin promoters

Pit-1 is a pituitary-specific transcription factor with protein expression limited to thyrotrope, somatotrope, and lactotrope cells of the anterior pituitary gland. We have recently described a thyrotrope-specific variant isoform of Pit-1, called Pit-1T, which contains an additional 14 amino acids in the activation domain generated by an alternate 3'-splicing choice. Pit-1T, in the presence of Pit-1, stimulates the thyrotropin beta-subunit (TSH beta) promoter in a thyrotrope-derived cell that lacks all Pit-1 isoform proteins. Three laboratories have identified another Pit-1 splice variant, called Pit-1 beta, which contains an additional 26 amino acids in the activation domain that is generated by a similar 3'-alternate splice choice. Pit-1 beta has been shown to stimulate the GH promoter, but not the PRL or TSH beta promoters. In this report, we evaluate the effect of the three Pit-1 isoforms (Pit-1, Pit-1T, and Pit-1 beta) on the GH, PRL, and TSH beta promoters when introduced into different cell types. The combination of Pit-1 and Pit-1T had a synergistic stimulatory effect on the TSH beta promoter, but not on the PRL or GH promoters in a thyrotrope-derived cell line that lacks all Pit-1 protein isoforms (alpha TSH cells). When added to GH3 cells, which lack only the Pit-1T isoform, Pit-1T selectively stimulated the TSH beta promoter and not the GH or PRL promoters, suggesting that the thyrotrope-specific Pit-1T exhibits a promoter-specific effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhancer-mediated high level expression of mouse pituitary glycoprotein hormone alpha-subunit transgene in thyrotropes, gonadotropes, and developing pituitary gland

The pituitary hormones LH, FSH, and TSH are heterodimers composed of a common alpha-subunit and unique beta-subunits. We demonstrate that 4.6, 2.7, 1.49 or 0.48 kilobases (kb) mouse alpha-subunit 5'-flanking sequences are sufficient for transgene expression in both gonadotropes and thyrotropes but not in inappropriate pituitary cells. In contrast, transgenes with bovine or human alpha-subunit flanking sequences have been shown to confer reporter gene expression only to gonadotrope cells, suggesting that the elements regulating cell-specific expression may differ between species. Equal levels of reporter gene expression were conferred by 5.0 and 0.48 kb in transiently transfected thyrotrope tumor-derived cells. In contrast, in transgenic mice, high level expression was only obtained with 4.6 kb 5'-flanking sequences, indicating the presence of an enhancer element between 4.6 and 2.7 kb. The 4.6 kb of 5'-flanking sequences are sufficient for both hormonal and developmental regulation of transgene expression. Mice rendered hypothyroid by radiothyroidectomy had significantly higher levels of transgene expression than either hyperthyroid or euthyroid animals. The temporal and spatial pattern of transgene expression in Rathke's pouch paralleled that of the endogenous gene; the onset of transgene expression occurred by embryonic day 9.5. Low level expression of both the transgene and the endogenous alpha-subunit gene were detected in some unexpected peripheral sites, such as the embryonic extraocular and olfactory regions, suggesting that alpha-subunit may have a more diverse role in development than previously considered.

A thyrotrope-specific variant of Pit-1 transactivates the thyrotropin beta promoter

Thyrotropin (TSH) beta is a subunit of TSH, the expression of which is limited to the thyrotrope cells of the anterior pituitary gland. Tissue-specific expression of the mouse TSH beta gene is conferred by sequences between -270 and -80 of the 5'-flanking region. We have investigated tissue-specific expression of the TSH beta promoter in two thyrotrope-derived cell types: 1) TtT-97 thyrotropic tumors, which express the endogenous TSH beta gene, and 2) an alpha-TSH cell line, which was generated from a thyrotropic tumor that has lost the ability to express the TSH beta gene. The pituitary-specific transcription factor Pit-1 is present in thyrotropes and interacts with three cis-acting elements in the functionally important region of the TSH beta promoter. Pit-1 protein is present in TtT-97 tumor cells but is absent from alpha-TSH cells. Reintroduction of Pit-1 into alpha-TSH cells by transient transfection does not restore TSH beta promoter activity. We have identified an alternately spliced variant of Pit-1, called Pit-1T, the mRNA and protein expression of which is limited to thyrotrope-derived cells. Pit-1T contains a 14-amino acid insert in the transactivation domain due to an alternate 3' splice acceptor site. Transiently transfected Pit-1T increases TSH beta promoter activity in TtT-97 thyrotropic tumor cells, whereas additional Pit-1 has no effect. The alpha-TSH cell line, which lacks all Pit-1 proteins, requires both isoforms in order to stimulate TSH beta promoter activity. These data suggest that Pit-1T is a thyrotrope-specific splice variant of Pit-1 that is required for TSH beta promoter stimulation; furthermore, both Pit-1 and Pit-1T are required for TSH beta promoter activity in thyrotrope cells.

Analysis of Pit-1 in regulating mouse TSH beta promoter activity in thyrotropes

TSH beta gene expression is restricted to pituitary thyrotropes. Since Pit-1 is present in these cells, we characterized Pit-1 RNA and protein in thyrotropes, and tested its function in regulating TSH beta promoter activity. We demonstrate that both TtT-97 thyrotropic tumors and pituitaries contain four Pit-1 transcripts of 3.2, 2.6, 2.4, and 1.9 kb, respectively. Only two transcripts of 2.7 and 2.1 kb were detected in alpha TSH cells, a thyrotrope derived cell that no longer expresses TSH beta. Western analysis revealed Pit-1 protein in TtT-97 cells but not in alpha TSH cells. DNase I protection assays localized Pit-1 binding to three areas of the mouse TSH beta promoter. However, basal TSH beta promoter activity was minimally stimulated when alpha TSH cells or TtT-97 thyrotropes were co-transfected with mouse Pit-1 and a mTSH beta luciferase construct. These studies suggest that Pit-1 is not limiting for cell-specific expression of the TSH beta gene in thyrotrope-derived cells and implies that additional thyrotropic factors are likely required.

Effect of thyroid hormone on T3-receptor mRNA levels and growth of thyrotropic tumors

Thyrotropic tumors (TtT97) contain mRNA transcripts for three T3-receptor (TR) isoforms, alpha 1, beta 1 and beta 2, and a non-receptor alpha 2-variant. We administered T4 (5 mg/l of drinking water) for one month to TtT97-bearing mice, to examine its effect on tumor growth and tumor TR isoform steady-state mRNA levels. Baseline mice were killed at the start of the experiment, and placebo mice were maintained hypothyroid. The treated tumors were 30-35% smaller than the baseline tumors (p = NS), while the placebo tumors were 2- to 7-fold larger than the baseline tumors (p < 0.05). TR beta 1 mRNA increased 5- to 6-fold, while TR beta 2 mRNA decreased by 76%. TR alpha 1 and the alpha 2-variant decreased by 52% and 70%, respectively. Therefore, the tumors decreased their growth rate in response to T4 administration, and increased the ratio of TR beta 1 to TR beta 2 mRNA. This raises the intriguing possibility of a correlation between the relative abundance of the TR beta isoforms and tumor growth.

Estrogen negatively regulates rat gonadotropin releasing hormone (rGnRH) promoter activity in transfected placental cells

To dissect the functional architecture of the rat gonadotropin releasing hormone (GnRH) gene promoter and its regulation by estrogen, gene transfer studies were performed in a placental cell line. 5'-Deletional constructs demonstrated that cis-acting elements important for rGnRH promoter activity in placenta were contained within a region of the proximal promoter between -73 and -16 bp upstream of the transcription initiation site. In addition, an inhibitory region was found from -903 to -424. Deletion of sequences to -424 which removed the inhibitory region, produced a promoter fragment which exhibited a 50% inhibition of GnRH promoter activity in the presence of estrogen (E) and cotransfected estrogen receptor. Negative regulation by E was retained in constructs deleted to -73 bp. We conclude that sequences important for placental cell expression and estrogen regulation of rGnRH lie within the region from -73 to -16. In addition, potential repressor sequences active in placental cells are present between -903 to 424.

Structure of the rat gonadotropin releasing hormone (rGnRH) gene promoter and functional analysis in hypothalamic cells

The gonadotropin releasing hormone (GnRH) gene encodes a protein which plays a critical role in mammalian reproductive physiology. Its expression is predominantly restricted to the hypothalamus although it has also been described in the placenta. To begin to determine the promoter elements important for tissue specific expression and to examine the mechanisms of developmental and hormonal regulation of the rat GnRH (rGnRH) gene, we cloned the rGnRH gene from a rat liver genomic DNA library. The nucleotide sequence of greater than 3 kb of 5'-flanking region was determined. The transcriptional initiation site in rat hypothalamic tissue and a mouse hypothalamic cell line were mapped by primer extension analysis and found to be different. In addition, transient transfection studies demonstrated that multiple regions of the distal promoter are important for tissue specific and basal promoter activity in hypothalamic cells. Furthermore, in these cells a potent activation region resides between -3026 and -1031 bp and suppressor region between -1031 and -903 bp upstream of the transcriptional start site. We conclude that different portions of the 5'-flanking region, which are activating and suppressing in nature, are critical for hypothalamic expression of the rGnRH gene.

A cell line that produces the glycoprotein hormone alpha-subunit contains specific nuclear factors similar to those present in thyrotropes

A unique characteristics of thyrotrope-specific gene expression is the coordinated expression and regulation of the alpha- and beta-subunits of TSH. A cell line (alpha TSH) derived from the transplantable mouse thyrotropic tumor MGH101A, which no longer expresses the TSH beta-subunit gene but continues to secrete large amounts of alpha-subunit, was used as a model to study alpha-subunit gene expression independent from the TSH beta-subunit gene and was compared with the expression in TSH-secreting TtT97 tumors. Transient transfection studies showed a striking similarity in the activity of 5' deletions of the mouse alpha-subunit gene promoter in both alpha TSH and TtT97 cells and localized two regions important for expression that spanned 100 base pairs, from -480 to -417 and from -417 to -381. These regions were found to have no activity in nonthyrotrope pituitary GH4 cells and L-cell fibroblasts. Analysis of the alpha-subunit 5' flanking DNA interactions with alpha TSH and TtT97 nuclear extracts showed two DNase I protected sequences, from -474 to -452 and from -447 to -400, both of which colocalized with the functionally important regions. Gel retardation analysis demonstrated the specificity of these interactions, and a similar migration of the DNA-protein complexes suggested that protein factors were similar in the two cell types. We conclude that the nuclear factors necessary for alpha-subunit expression in thyrotropes are retained in alpha TSH cells. Moreover, since alpha TSH cells do not express the TSH beta-subunit gene, the factors that determine the expression of the alpha-subunit may not be sufficient for TSH beta-subunit gene expression.

The NPEY sequence is not necessary for endocytosis and processing of insulin-receptor complexes

The tetrameric amino acid sequence AsnProXTyr (NPXY), where X represents any amino acid, is conserved in the intracytoplasmic domains of several membrane proteins and has been postulated to play a role in receptor-mediated endocytosis. The human insulin receptor (hIR) contains a single copy of the sequence AsnProGluTyr (NPEY) in its intracytoplasmic domain. To determine if this putative consensus sequence is necessary for endocytic functions of hIR, we constructed a mutant receptor, hIR delta NPEY, that lacks NPEY sequence, stably expressed this mutant receptor in Chinese hamster ovary cells, and then studied its endocytic functions. When compared to wild type hIR similarly expressed in Chinese hamster ovary cells, the hIR delta NPEY mutant exhibited: 1) normal subunit organization and insulin binding affinity; 2) essentially normal internalization of covalent photoaffinity labeled insulin-receptor complexes; and 3) normal internalization of receptor-bound [125I]insulin as well as normal degradation and release of the internalized insulin. Therefore, we conclude that the NPEY sequence in the juxtamembrane domain of hIR is not necessary for its endocytic function.

Isolation and characterization of mouse complementary DNAs encoding alpha and beta thyroid hormone receptors from thyrotrope cells: the mouse pituitary-specific beta 2 isoform differs at the amino terminus from the corresponding species from rat pituitary tumor cells

Thyroid hormones (T3) and their receptors (TR) play a critical role in the function of the pituitary gland, particularly in thyrotropes, where they regulate expression of the alpha- and beta-subunits of TSH. Since the pituitary gland is composed of several cell types, we undertook a characterization of TR subtypes in a murine thyrotropic tumor (TtT-97), an excellent model in which to study thyroid hormone action in thyrotropes. We screened a thyrotrope cDNA library with rat TR alpha 1 and TR beta 1 cDNA probes and isolated cDNAs encoding the mouse TR alpha 1 and TR beta 1 isoforms as well as a partial clone corresponding to the non-T3 binding carboxy-terminal alpha 2 variant. The polymerase chain reaction was used to amplify additional cDNAs for the specific 5' domains of the mouse TR beta 1 and the pituitary-specific TR beta 2 amino-terminal variant. Using hybridization probes that discriminate between the alpha and beta isoforms and their variants, we demonstrated that thyrotropes contain TR alpha 1 and alpha 2 mRNAs as well as transcripts encoding Rev-erbA, which arise by transcription from the opposite strand of the TR alpha gene. In thyrotropes, the ratio of alpha 2 to TR alpha 1 mRNA levels more closely resembled the distribution in mouse brain than that in heart, where the mRNA levels of TR alpha 1 and alpha 2 are comparable. TR beta 1 and TR beta 2 mRNAs were detected in thyrotropes and were of similar size (approximately 6.4 kilobases). Despite the almost complete conservation between the rat and mouse TR beta 1 sequences at the protein level, the mouse and rat TR beta 2-specific N-terminal domains were less conserved, and the mouse protein was shorter by 39 amino acids at the N-terminus. Of the receptor species, only the mRNA encoding the TR beta 2 isoform, which was restricted to thyrotropes, was decreased by T3 treatment, although the mRNA for the alpha 2 variant was also reduced by T3 in thyrotropes and heart tissue. Levels of TR beta 1 mRNA were not changed in liver, but were increased in thyrotropic tumors and also somewhat in brain, an organ that is not responsive to T3 by classical criteria.

Insulin downregulates the steady-state level of its receptor's messenger ribonucleic acid

The effects of insulin on the steady-state level of human insulin receptor (hIR) mRNA were examined in the HepG2 human liver cell line using Northern blot analysis of either total cellular or poly(A)+ RNA. In control cells, up to six (4.5, 5.2, 7.4, 8.5, 9.4 and 10.8 kb) hybridizable species of hIR mRNA were identified, with the 8.5 and 10.8 kb species being most prominent. Incubation for 18 hrs with 1 microM insulin resulted in a similar decrease (to approximately 35% of control) of all the hIR mRNA species. The insulin effect was dose-dependent and was half-maximal by 2-3 hrs and maximal by 4-6 hrs of incubation at 37 degrees C. The hIR mRNA levels remained maximally insulin suppressed for up to 18 hrs but thereafter the effect became attenuated. These results indicate that insulin downregulates the level of hIR mRNA with a biphasic time-course and that this process is most likely part of the general mechanism by which insulin maintains the homeostatic control of its cellular receptor levels.

Protein factors in thyrotropic tumor nuclear extracts bind to a region of the mouse thyrotropin beta-subunit promoter essential for expression in thyrotropes

The beta-subunit gene of TSH is specifically expressed in thyrotrope cells of the anterior pituitary gland. To define the particular TSH beta-subunit gene sequences responsible for tissue-specific expression, TSH beta promoter fragments were assessed for promoter activity by gene transfer into TSH-expressing thyrotropic tumor cells (TtT-97). Previous studies have shown that the murine TSH beta gene promoter was more efficiently used in TtT-97 cells compared to other pituitary-derived cells or nonpituitary fibroblasts and that a 191-basepair DNA sequence of the 5' flanking region between -271 and -80 was sufficient for maximal promoter activity in thyrotropes. Further deletional analysis within this region has localized the area responsible for expression in thyrotropes to a 37-basepair region between -117 and -80 up-stream of the major transcriptional initiation site. DNase-I protection assays demonstrated that this functionally defined 5' flanking area, in addition to the adjacent sequences immediately up-stream and down-stream, interacts with protein factors present in nuclear extracts from TtT-97 tumor cells. When fused to a heterologous promoter, fragments derived from the region between -271 and -80 exhibited cell-specific activity, although this was not conferred solely by the TSH beta promoter fragment from -117 to -80. Heterologous promoter activity was further stimulated when fragments containing the areas from -271 or -201 to -77 were used, suggesting combinatorial cis interactions between these regions of the TSH beta promoter. DNase-I protection studies suggest that there are multiple protein-binding domains in the mouse TSH beta 5' flanking sequence. Only the more proximal domains, which encompass important promoter elements, appear to be required for efficient expression in thyrotropes, whereas other more up-stream sites of protein interaction may be involved in regulatory aspects of TSH beta gene expression.

Activation of the murine thyrotropin beta-subunit promoter by GH4 rat pituitary cell-free extracts

Expression of the TSH beta subunit gene is restricted to the thyrotroph cells of the anterior pituitary. Previously we identified several AT-rich DNA elements within the murine (m) TSH beta 5'-flanking region, denoted as D1 (-253 to -227), P4 (-142 to -131), P3 (-126 to -112), P2 (-106 to -98), and P1 (-76 to -68) which bind thyrotroph-specific factor(s). These sites are related to, but distinct from GHF-1 and LSF-1 binding sites, which restrict GH and PRL gene expression to pituitary somatotrophs and lactotrophs, respectively. To determine whether different pituitary cell types contain related factors capable of activating the mTSH beta promoter, cell-free transcription studies were performed using extracts from GH4 rat pituitary somatomammotroph cells. AI-through the endogenous mTSH beta gene is not expressed in GH4 cells, in vitro transcription of the mTSH beta promoter, normalized to the Rous sarcoma virus internal control, revealed faithful transcription initiation from the authentic mTSH beta CAP sites in GH4 but not in HeLa cell extracts. Cell-free transcription analysis of mTSH beta 5'-deletion mutants revealed consistent promoter activity with deletion to position -46 but complete loss of activity when deleted to position -9. To better define the specific factors in pituitary somatomammotrophs which interact with and activate the mTSH beta promoter, DNase I protection and gel-shift studies were performed using extracts from GC rat pituitary somatomammotroph cells and DNA affinity-purified lactotroph-specific transcription factor, LSF-1, required for rat PRL promoter activity, and purified from GC cells. These cells contain a factor(s) which binds to thyrotroph-specific elements of the mTSH beta promoter. These studies also show that LSF-1 binds the D1 and proximal thyrotroph-specific elements of the mTSH beta promoter and is capable of reconstituting the trans-activation of the mTSH beta promoter in HeLa nonpituitary cell extracts in vitro. Conversely, nuclear factors present in TtT-97 murine thyrotrophs bind the proximal lactotroph-specific elements on the rPRL promoter. This in vitro transcription assay provides a means to biochemically dissect the trans-activation of the mTSH beta promoter and to determine the functional overlap of distinct pituitary cell-specific factors in regulating GH, PRL, and TSH beta gene expression.

5'-Heterogeneity in human progesterone receptor transcripts predicts a new amino-terminal truncated "C"-receptor and unique A-receptor messages

Human progesterone receptors (PR) are thought to comprise two naturally occurring hormone-binding proteins: 94-kDa A-receptors and 120-kDa B-receptors. In this paper we present evidence for a third human PR, an N-terminally truncated, 45- to 50-kDa species, termed the C-receptor. To determine the translational origin of B- and A-receptors we mapped the multiple messages that code for human PR by Northern blot analyses, using a series of oligonucleotides and cDNA fragment probes corresponding to different regions of the PR message. In addition to the six transcripts of 2.5, 3.2, 4.5, 5.2, 6.1, and 11.4 kilobases (kb) originally described, we found that the 11.4-kb species is a complex of four bands that we have termed I-IV. Analysis of poly(A)+ RNA derived from T47Dv human breast cancer cells using a variety of 5'-specific probes has identified three separate structural classes of human PR transcripts, indicating extensive 5'-termini heterogeneity. Class A messages, the 2.5- and 5.2-kb species, lack the sequences surrounding AUGB (codon 1), which is the translation initiation site for B-receptors, but contain AUGA (codon 165), the initiation site for A-receptors, and, therefore, potentially encode only the latter. Class B messages, consisting of the 3.2-, 4.5-, and 6.1-kb species as well as bands I and II of the 11.4-kb complex contain both AUGB and AUGA and could encode both receptor forms.(ABSTRACT TRUNCATED AT 250 WORDS)

T47DCO cells, genetically unstable and containing estrogen receptor mutations, are a model for the progression of breast cancers to hormone resistance

We postulate that one mechanism for the progression of breast cancers to hormone resistance involves the acquisition of mutant estrogen receptors (ER)4 by genetically unstable cell subpopulations. The T47D human breast cancer cell line may be a model for such progression, having sublines that are ER positive and estrogen responsive, ER positive and estrogen resistant, or ER negative. Also, T47D cells can be either hyperdiploid (HD) or hypertetraploid (HT) or persistently alternate between these states. T47DCO cells are a HD and ER-positive, but estrogen-resistant, subline of T47D cells that undergoes spontaneous tetraploidization. Such a stable variant, designated T47Dv, is 85% HT (Cancer Res., 49: 3943, 1989). We now show that single-cell clones derived from the mixed HD/HT T47Dv can be either HD or HT, and can be either estrogen responsive or estrogen resistant, for growth and for progesterone receptor regulation. To begin the study of ER in this model system of T47DCO and their derivatives, we have generated complementary DNA libraries from the parental HD T47DCO cells and have isolated three ER complementary DNA mutants. These include two frame-shift/termination mutants that would encode ERs truncated in the DNA-binding domain and in the hormone-binding domain and a third mutant with a large in-frame deletion spanning the hinge region and a part of the hormone-binding domain. If expressed, these mutant ERs would lack hormone-binding capacity and would be undetected by the anti-ER antibodies currently in clinical use. Genetic instability, when associated with mutant ERs in subpopulations of breast tumor cells, may provide the selective pressure leading to hormone resistance. T47DCO cells and their clonal derivatives provide a model for the systematic study of ER mutations and other mechanisms of hormone resistance in Stage IV breast cancer.

Lipoprotein lipase gene expression in rat adipocytes is regulated by isoproterenol and insulin through different mechanisms

Lipoprotein lipase (LPL) is highly regulated by catecholamines and insulin in adipocytes. Isoproterenol, a beta-adrenergic agonist, decreases LPL enzyme activity, whereas insulin increases LPL activity. We have isolated an 868-basepair rat LPL cDNA clone to assess hormone-mediated changes in LPL steady state mRNA levels and LPL gene transcription rates in adipocytes. Northern blot analysis of isoproterenol-treated (10(-6) M) adipocytes showed that LPL steady state mRNA decreased by 15 min. Nuclear run-on transcription assays in isoproterenol-treated cells indicated that LPL gene transcription was also decreased at 15 min compared to that in control cells. Conversely, insulin (6.7 x 10(-8) M) mediated an increase in LPL steady state mRNA in treated adipocytes, yet LPL gene transcription was not different from that in control cells. Thus, the isoproterenol-mediated decrease in LPL enzyme activity and steady state mRNA levels in adipocytes is associated with decreases in LPL gene transcription. Insulin, which does not affect LPL gene transcription, increases LPL enzyme activity and steady state mRNA levels. The effect of insulin on LPL mRNA is probably due to insulin-induced changes in mRNA stability.

Thyrotrope expression and thyroid hormone inhibition map to different regions of the mouse glycoprotein hormone alpha-subunit gene promoter

The alpha-subunit gene of the glycoprotein hormones is normally expressed in pituitary thyrotropes and gonadotropes and in placental cells. Thus, this gene must contain elements that mediate expression and hormonal responses in different cell types. The localization of DNA regions important for expression and regulation of the alpha-subunit gene in thyrotrope cells has not previously been reported. In these studies luciferase expression constructs containing 1700 basepairs of 5' flanking DNA derived from the mouse alpha-subunit gene were introduced by electroporation into freshly dispersed cells from TSH-producing mouse pituitary tumors (TtT 97). This promoter functioned with greater efficiency in thyrotropes than in nonthyrotrope pituitary GH4 cells and L-cell fibroblasts. Primer extension confirmed that transcription from the alpha-subunit constructs initiated at the same site as the endogenous gene. Studies using 5' truncations showed a progressive loss of alpha-subunit promoter activity in thyrotropes between -480 and -120, with regions upstream of -254 contributing substantially to expression in thyrotrope cells. Thyroid hormone inhibited alpha-subunit promoter activity in a dose-dependent fashion, although in vivo treatment of tumors with thyroid hormone before transfection was necessary to achieve maximal inhibition. Thyroid hormone inhibition of alpha-subunit promoter activity also occurred in GH4 cells, but no effect was observed in L-cells. Studies using 5' truncations localized a region responsible for thyroid hormone inhibition between -62 and +43, encompassing the TATA sequence and the transcriptional initiation site. When this region was compared to the thyroid hormone inhibitory regions of the alpha-subunit genes from other species and the mouse TSH beta-subunit gene, a 6-basepair motif, 5' (G/A)GTG(G/A)G 3', emerged as a possible consensus sequence for a thyroid hormone inhibitory element.

TSH subunit gene promoters from a murine alpha-subunit producing tumor function normally

The murine thyrotropic MGH101A tumor is characterized by absent thyrotropin (TSH) beta gene expression and altered thyroid hormone (T3) regulation of the alpha-subunit. Comparison of the promoter structures of both alpha and TSH beta subunit genes from MGH101A with the promoter in expressing TtT-97 thyrotropes revealed no detectable differences. Transfection of the TSH beta promoter from MGH101A linked to luciferase showed minimal expression in primary or cloned MGH101A cells, or L-cells. However, a 6- to 10-fold increase in expression was exhibited in transfected thyrotropes. For the alpha gene, promoter activity was highest in thyrotropes and in cloned MGH101A cells, 5-fold lower in MGH101A tumors, and 10-fold lower in L-cells. Both promoters were not substantially affected by T3 treatment in MGH101A cells. In thyrotropes, promoter activity was inhibited 62.5% and 57.7% by 10 nM T3 treatment for the TSH beta and alpha genes, respectively. DNase I protection showed that factors from TtT-97 but not from MGH101A cells interacted with regions in the TSH beta promoter, while nuclear extracts from each tumor demonstrated at least one protein-DNA interaction with the alpha-subunit promoter. These studies suggest that the molecular defects in the MGH101A tumor are related to the absence of trans-acting factors and are not a result of altered primary gene structure.

Identification of cis-acting promoter elements important for expression of the mouse glycoprotein hormone alpha-subunit gene in thyrotropes

The glycoprotein hormone alpha-subunit gene is expressed in a cell-specific manner in the anterior pituitary and placenta. Previous studies have shown that the region between -178 to -111 is indispensable for placental-specific expression of the human alpha-subunit gene. Using gene transfer techniques with chimeric luciferase plasmids, this report identifies regions of the mouse alpha-subunit promoter that are important for transcriptional activation in primary thyrotropic cells. Transient expression of a series of 5' flanking DNA deletions resulted in stepwise reductions of basal promoter activity between -480 to -417 (4-fold), -254 to -177 (5-fold), and -177 to -120 (3.5-fold). DNase-I protection analysis with nuclear extracts from thyrotropic tumor cells revealed specific protein-DNA interactions within each of these functionally defined regions. These were mapped to positions -474 to -452, -447 to -419, -213 to -170, and -158 to -101 within the 5' flanking region. In contrast, in mouse fibroblast L-cells no significant difference in alpha-subunit promoter activity was found by deleting the region from -480 to -177. However, a 3-fold decrease, similar to that found in primary thyrotropes, was found by deleting the region from -177 to -120. Further, a smaller region between -138 and -122 was the only area detected by the DNase-I protection assay using L-cell nuclear extracts. Thus, several cis-acting promoter elements located up-stream of position -177 are important for expression in thyrotropes. These elements also bind nuclear factors present in thyrotropes but not in nonpituitary fibroblasts and, therefore, differ from those mediating expression of the human alpha-subunit gene in the placenta.

Thyroid hormone regulates the mouse thyrotropin beta-subunit gene promoter in transfected primary thyrotropes

In TtT 97 cells, a thyrotropin-producing mouse pituitary tumor, thyroid hormone rapidly inhibits the transcription rate of both the thyrotropin alpha- and beta-subunit (TSH beta) genes, and this closely parallels the increase in nuclear thyroid hormone receptor occupancy. In this study, we have identified regions of the mouse TSH beta gene which are involved in mediating tissue-specific and thyroid hormone-regulated expression. Transient expression studies were performed using a series of chimeric plasmids in which 5'-flanking DNA was ligated to the firefly luciferase gene. Following transfection by electroporation, efficient expression of TSH beta 5'-flanking luciferase constructs occurred only in cells derived from TtT 97 tumors which express the endogenous TSH beta gene. Deletion analysis demonstrated that the region of the 5'-flanking DNA between positions -271 and -80 relative to the major transcriptional start site is important for TSH beta promoter activity in thyrotropes. No expression was measurable in mouse L cells, a fibroblast line, whereas a low level of expression was seen in MGH 101A cells derived from a thyrotropic tumor which no longer expresses the TSH beta gene. Reduced expression of TSH beta constructs was also found in GH3 and GH4 pituitary tumor lines. Addition of thyroid hormone effectively inhibited the level of transient TSH beta promoter activity in TtT 97 cells in a dose-dependent manner. The inhibitory effect was more pronounced and more accurately reflected the transcription rate data when transfected cells were derived from tumors treated with thyroid hormone for 5 days prior to transfection. Deletion of all but 46 base pairs of TSH beta gene 5'-flanking DNA and 3 base pairs of the first exon had no effect on thyroid hormone inhibition. This indicates that signals sufficient for transcriptional regulation of the TSH beta gene by thyroid hormone reside in the vicinity of the proximal promoter and may act by interfering with basal transcriptional factors.

Identification of thyrotroph-specific factors and cis-acting sequences of the murine thyrotropin beta subunit gene

Pituitary thyrotroph cells specialize in the synthesis of TSH, and thus represent a model to study cell-specific gene expression. We have used the murine TSH beta (mTSH beta) gene promoter and TSH-producing and nonproducing transplantable tumors derived from murine thyrotroph cells, referred to as TtT-97 and MGH 101A, respectively, to identify nuclear factors which selectively interact with the mTSH beta gene. DNase I protection analyses demonstrate that factors present in TtT-97 nuclear extracts bind with high affinity to five separate sites in the TSH beta promoter region, denoted as distal D1 (-253 to -227) and proximal, P1 (-76 to -68), P2 (-106 to -98), P3 (-126 to -112), and P4 (-142 to -131) footprints. By contrast, non-TSH beta expressing thyrotroph cell nuclear extracts and L-cell nonpituitary cell extracts did not appear to footprint the D1 site; whereas the nonpituitary nuclear extracts revealed minimal DNase I protection in the P1-P4 regions. These data show that the distal D1 site is thyrotroph specific and contains a 6 base pair direct repeat sequence (5'-AGATAT-3'). Factor occupancy of the D1 site is protein dependent, occurs rapidly (less than 15 sec), is destabilized by 170 mM KCl, and results in an associated DNase I hypersensitive region. A double-stranded oligonucleotide spanning the D1 footprint competes only the distal factor binding region. Transfection of plasmid constructs containing progressive 5'-deletions of the mTSH beta promoter linked to the reporter gene luciferase into primary TtT-97 cells demonstrate a marked decrease in activity between the regions -270 and -79, which contains the D1 region.(ABSTRACT TRUNCATED AT 250 WORDS)